The present invention relates to static discharge devices and, more particularly, to machinery components having static electricity dissipative qualities.
In industrial settings, vacuum systems are widely used to pneumatically transfer bulk solids from one container to another, to meter solids from a container into a process, and to sweep up powder spills, leaks or dust accumulations. A flexible hose is commonly attached to the vacuum unit and at the end of the flexible hose is a rigid tubular device (cylindrical or polygon in shape) typically referred to as the vacuum wand. The wand is used by a human operator to direct the point of entry into the vacuum system where the accumulation of powder, bulk solids, or dust may be greatest. Metallic wands made from steel, stainless steel, aluminum, copper, or other metals are normally used because of their rigidity and their ability to be grounded by a wire to earth ground. Polymeric wands are not used, even though they are rigid enough to function as a wand, because of their characteristic of developing a large static electric charge which does not transfer to earth ground.
Static electric charge can build up on any isolated ungrounded material or device. In a vacuum or pneumatic conveying system, particles transfer electrons when they come in contact with other particles or materials. This is referred to as triboelectric charging. The amount of charge and the type of charge (positive or negative) depends on the speed of contact, the intimacy of contact, and the relative position of the two materials in the triboelectric series.
Polymeric materials (plastics) are normally used as electrical insulators because of their ability to prevent a flow of electrons through the bulk of the material. Insulative materials have surface resistivities greater than 10.sup.14 ohms per square (e.g. cm.), and volume resistivities greater than 10.sup.14 ohms-cm. The high molecular weights of polymers and their amorphous or low crystalinity molecular structure allow pockets of electrons to accumulate and not to evenly distribute electrons across the surface or through the bulk of the material. As a result, earth grounds attached to vacuum wands made with insulative polymeric materials are not effective in allowing the static charge build up to flow to earth ground. On the other hand, metals have surface resistivities of about 10.sup.-3 ohms per square (e.g. cm.)and volume resistivities near 10.sup.-3 ohms-cm which is considered in the category of electrical conductors and this allows instantaneous discharge of the entire static charge build up throughout the wand to flow to earth ground.
A shortcoming of metallic wands however is that earth grounds are 1) not connected at all; 2) not connected properly; 3) the conductive wire breaks at a point away from the wand; or 4) the conductive wire is so long to earth ground or too small in diameter to the point that the static charge being generated at the wand can more easily discharge itself directly off the wand to the operator or another grounded device. Static electric shocks to operators are common experiences and are painful. Static electric sparks can be an ignition source in an atmosphere with an explosive concentration of dust particles and may cause a hazardous explosion. Metal wands are so conductive that in the event of improper grounding, the entire amount of static charge, which can easily reach 10,000 volts in a 48" long by 2" diameter wand, is released instantaneously in a few milliseconds. This represents an energy release of 24 millijoules which is above the minimum ignition energy value for many powders and dusts. Humans are sensitive to static electric shocks above 3,000 volts. So an ungrounded or improperly grounded metallic vacuum wand can be a nagging source of operator pain and can represent a safety hazard when conditions are just right for a dust explosion. Accordingly, there is a need for an entry tube or vacuum wand that will not be a hazard, even when not properly grounded.